Presently, there are numerous methods used to coat or apply a liquid onto a surface. These methods include traditional brush painting, dipping, roller-type transfer, spraying, extruding and film application. Roller type transfer application is still popular, but this method invariably results in some nonuniformity in coating and residue on the roller. The spraying method is perhaps the most frequently used of these methods, due to its simplicity and the ease in achieving an applied film which is relatively thin and uniform in thickness.
However, even spraying is not perfect. Generally, spray coating produces some uneven film thickness from the center of the spray pattern to the outer portions of the pattern. Additionally, because of the high pressures used, spraying produces an environmental problem and is not very efficient, due to the high number of airborne particles or droplets which deflect and disperse off of the surface to be coated.
It is an objective of this invention to improve the uniformity in thickness of a liquid coating applied to a surface.
It is another objective of the invention to more efficiently spray coat a surface by reducing dispersion.
The above-stated objectives are achieved by filling the pores of a porous sheet with a liquid and then pushing the liquid out of the pores by pressure to transfer the liquid onto the surface to be coated. Preferably, the liquid is pushed out of the pores by spraying a compressed fluid.
By wiping the excess liquid from the sheet prior to transfer, only a volume of liquid equal to the volume of the pores will remain. If the volume of the pores is known, the volume of the liquid transferred to the surface will be known with a high degree of accuracy.
Variations of this method include arranging the size and/or the distribution pattern of the pores in the sheet to form a recognizable pattern. This technique can be used for printing to achieve a shading effect normally produced by silk screening. Also, the pores may be tapered in the direction of the surface to be coated.
This invention makes it possible to spray a film evenly on an entire surface to be coated, regardless of the desired thickness of the coating, while also eliminating worries about dispersion and/or diffusion of the sprayed material. The liquid transfered to the surface is uniform if all the pores are identical in size and evenly distributed on the sheet. The width and length of the spray pattern are also easy to control, because they are determined by the dimensions of the sheet.
By locating a compressed air nozzle in contact with one side of the sheet and spraying compressed air into the sheet, the liquid inside the pores will be sprayed toward the surface to be coated, which is located on the opposite side of the sheet. Parallel movement of the nozzle with respect to the sheet and the surface will expel all of the liquid held in the pores of the sheet. The thickness or density of the sprayed film may be controlled by also moving the surface to be coated, relative to the sheet, but at a different speed than the nozzle. According to another variation, if the nozzle is moved relative to a screen and a surface which are fixed in parallel position, and the flow of the compressed air to the nozzle is controlled, i.e., turned off and on according to a desired sequence, the liquid can be sprayed onto the surface to form a pattern. If it is desired to reduce the stickiness or adhesion of the liquid, the compressed air may be heated. If the liquid includes a solvent subject to evaporation, the solvent may be added to the compressed air.
The present invention contemplates three preferred apparatus for performing the above-described spraying methods, wherein the porous sheet may be flat, in the form of an endless belt or in the form of a hollow cylinder. Each of these embodiments automates the process steps of filling the porous sheet with a liquid, wiping excess liquid from the sheet and spraying the sheet to transfer the liquid to a surface. Each of these embodiments also facilitates relative movement between the nozzle and the sheet and the surface to be coated.